The present invention relates to a method and apparatus for automatically adjusting the transmit power level of data communication equipment (DCE) located at a telecommunication subscriber premises and, more particularly, to a method and apparatus for adjusting the transmit power level of a DCE located at a telecommunication subscriber premises in response to the detection by the DCE of an off-hook signal generated by a telephone which is located at the telecommunication subscriber premises and which is connected to the same copper pair as the DCE.
Conventional communications systems typically include two DCEs connected across a communication connection, such as a subscriber loop associated with a public-switched-telephone-network (PSTN). The communication connection is comprised of a pair of copper wires that are terminated at the DCE so that the DCEs may communicate with each other via either half-duplex or full-duplex communications. These DCEs normally include a voltage driver that has a low source impedance for driving the data signal across the connection. Consequently, to effect multipoint communication, bridging additional voltage-driven devices to an end of the communications connection significantly loads the connection making it difficult to successfully communicate across the connection.
As a result of the difficulty of bridging multiple voltage-driven DCE tributaries to the connection, in order to increase the number of DCEs at a particular location, prior art systems typically increase the number of communications connections such that each tributary at a customer premises is serviced by a separate communications connection. The insertion of additional connections into the system increases the overall cost of the system, particularly when the connection is a subscriber loop associated with a PSTN.
Recently, a multipoint system has been developed by the assignee of the present application that allows a plurality of tributaries to be bridged to a standard PSTN telephone twisted pair. The DCEs coupled to each of the tributaries communicate with a DCE located at the central office via a particular polling protocol. Each of the DCEs coupled to the tributaries comprises a current source-driven (i.e., driven by a high-impedance source) analog front end circuit that couples each DCE to its respective tributary via a dry transformer.
In this type of multipoint environment, typically one or more DCEs and one or more telephones are connected to the tributaries which, in turn, are connected to the standard PSTN twisted pair. Since the telephones and DCEs are connected to the same twisted pair, it is possible in some situations for the subscriber to hear the DCEs when the subscriber has his or her ear to the telephone ear piece. This may be due to the fact that telephones are manufactured by many different manufacturers and they often have varying nonlinear distortion properties. Therefore, some telephones will intermodulate the multi-carrier, or Quadrature Amplitude Modulation (QAM), DCE transmit power producing audible noise will be heard by the subscriber.
In order to achieve the highest transmission performance possible, it is desirable to configure DCEs to transmit at the highest possible transmit power level. However, DCEs are capable of successfully transmitting information with high performance even when the transmit power level is less than maximum. Therefore, in multipoint environments, it would be desirable to have a way to adjust the transmit power level of DCEs depending on whether or not a telephone is currently being utilized, i.e., depending on whether or not a telephone is off-hook. It would be particularly desirable to have a DCE which is capable of detecting an off-hook signal and of automatically adjusting its transmit power to a level which is suitable for the telephone which has gone off-hook.
Accordingly, a need exists for a DCE which can be employed in a multipoint environment, which is capable of detecting when an off-hook condition has occurred, and which is capable of adjusting its transmit power level to a level which is appropriate for the telephone that has gone off-hook.
The present invention provides a method and apparatus for automatically adjusting the transmit power level of data communication equipment (DCE) being used in a multipoint environment. The apparatus of the present invention preferably is comprised in a DCE. The DCE comprises a receiver which receives signals transmitted to the DCE over a transmission medium and a transmitter which transmits signals from the DCE over the transmission medium, which may be a twisted wire pair of a digital subscriber loop. The DCE also comprises a memory device which stores equalizer tap signatures and transmit power levels and a processor which controls the operations of the DCE and which is in communication with the receiver, the transmitter and the memory device.
Each transmit power level stored in the memory device is associated with a particular equalizer tap signature stored in the memory device. In a multipoint environment, at least one telephone and at least one DCE are connected to the same twisted wire pair. When a telephone connected to the twisted wire pair goes off-hook, the receiver notifies the processor that the telephone has gone off-hook. Off-hook detection can be achieved via DC voltage detection across tip and ring or other means. The receiver comprises an equalizer which generates an equalizer tap signature based on the off-hook dial tone of the telephone. The receiver equalizer then delivers a tap signature associated with the off-hook impedance to the processor. The processor compares the equalizer tap signature associated with the off-hook condition with equalizer tap signatures stored in the memory device until an equalizer tap signature has been found which matches (within a prescribed accuracy or tolerance) the equalizer tap signature delivered to the processor from the receiver. When the processor determines that a match has occurred, it then reads the transmit power level associated with the matching equalizer tap signature out of the memory device and sets the transmit power level of the transmitter to the transmit power level read out of the memory device.
In accordance with the preferred embodiment of the present invention, the transmit power levels are stored in the memory device in a lookup table. Each equalizer tap signature stored in the memory device is associated with a respective address in the lookup table. When the processor finds a matching tap signature, the processor uses the matching tap signature to obtain the corresponding address in the lookup table. The transmit power level associated with the matching tap signature is then read out of the lookup table by the processor and the transmitter is set to the transmit power level.
In accordance with the preferred embodiment of the present invention, the tap signatures stored in the memory device are generated by the equalizer during an initialization phase during which a telephone connected to the line is placed in an off-hook condition. Usually upon system initialization, the user who is operating the telephone enters a code on the telephone keypad which the processor detects and utilizes to reduce the transmit power level of the transmitter until the user determines that the transmit power is at a suitable (quiet) level. The processor obtains the tap signature from the receiver which corresponds to the transmit power level after it has been adjusted by the user to a suitable level. The processor stores the tap signature corresponding to the suitable transmit power level in the memory device at a predetermined location and then stores the corresponding transmit power level at a predetermined location in the lookup table in the memory device.
This initialization phase is performed for each telephone and for each DCE connected to the twisted wire pair. Subsequent to the initialization phase, any time a telephone goes off-hook, the processor recognizes the tap signature of the off-hook telephone and causes the transmitter to be set to the appropriate transmit power level. In addition, or in conjunction with adjusting transmit power level, transmit spectrum may be shaped or shifted in frequency to reduce inter-modulation produced audio noise.
These and other features and advantages of the present invention will become apparent from the following discussion, drawings and claims.